1. Field of the Invention
The present invention relates mainly to a Schottky barrier type semiconductor device such as a Schottky barrier diode, Schottky barrier collector type transistor, Schottky barrier gate type field effect transistor and the like, and particularly to a Schottky barrier type semiconductor device whose breakdown voltage characteristics are improved.
2. Prior Art of the Invention
In a prior art Schottky barrier diode, as shown in FIG. 1 an N-type silicon semiconductor substrate 1 of, for example, low impurity concentration is normally deposited thereon with a metal contact 2 composed of a metal capable of forming a Schottky barrier therewith, for example, aluminum, thus a Schottky barrier 3 being formed. In this illustrated example, the semiconductor substrate 1 is composed of an N-type original substrate 4 having high impurity concentration and an N-type semiconductor layer 5 having relatively low impurity concentration which is formed on the original substrate 4 by epitaxial growth process. On the semiconductor layer 5 is deposited the metal contact 2 to form the Schottky barrier 3 therewith. In the Schottky barrier diode constructed as mentioned above, an insulating layer 6 made of silicon dioxide (SiO.sub.2) is deposited on the surface of the substrate 1 as a protecting layer for surface passivation. The insulating layer 6 is provided with a window 6a through which the metal contact 2 is selectively deposited on the semiconductor layer 5 and also the peripheral edge 3a of the Schottky barrier 3 is covered by the insulating layer 6. Reference numeral 7 denotes an electrode deposited on the original substrate 4 in an ohmic contact therewith, that is, a cathode electrode in the illustrated example.
With the construction as mentioned above, however, there is a defect that the breakdown voltage (inverse voltage) of the Schottky barrier 3 is relatively low because of great field concentration at the peripheral edge 3a of the Schottky barrier 3. Particularly, when the insulating layer 6 made of SiO.sub.2 is formed as the protecting layer for surface passivation as mentioned above, a distortion caused by great difference of thermal expansivity between the above silicon dioxide SiO.sub.2 and, for example, silicon Si in the substrate 1 or semiconductor layer 5 is concentrated to the inner peripheral edge of the discontinuous portion of the insulating layer 6 or the window 6a, that is, to the peripheral edge 3a of the Schottky barrier 3. Therefore, when the barrier 3 is applied with an inverse voltage, a depletion layer expanding therefrom into the substrate 1 becomes narrow at the peripheral edge 3a of the barrier 3 as shown by a chain line a in FIG. 1 to cause therein the field concentration with the result that the breakdown is apt to occur at the aforesaid edge portion.
In the Schottky barrier diode as mentioned above, it has been proposed to prevent the breakdown at the peripheral edge 3a of its Schottky barrier 3. That is, as shown in FIG. 2 an annular P-type region 8 of different conductivity type from that of the substrate 1, that is, a so-called guard-ring is formed along the peripheral edge 3a of the Schottky barrier 3 so that a depletion layer may expand outside the region 8 as shown by a chain line a, thus the effect due to the distortion at the inner peripheral edge portion of the window 6a of the insulating layer 6 is avoided.
Further, it has been proposed that as shown in FIG. 3 a tapered portion 6b is provided at the peripheral edge of the window 6a of the insulating layer 6 and the depletion layer at the peripheral edge 3a of the barrier 3 is expanded as shown by a chain line a according to the field effect caused by a voltage applied to the metal contact 2.
However, a work of providing the guarding region 8 or providing the tapered portion 6b at the peripheral edge of the window 6a of the insulating layer 6 is troublesome. Further, when the region 8 is provided, the total area becomes large.
Meanwhile, when a silicon dioxide layer as described above is formed on the surface of a semiconductor substrate as the insulating layer for passivation, a positive electric charge, for example, natrium ion Na.sup.+ is contained therein to cause a memory operation so that the surface of the semiconductor substrate becomes unstable. In addition, when the substrate is of P-type by way of example, an N channel is produced therein to cause the decrease in breakdown voltage. Further, when a resin mold is applied on a semiconductor pellet, the polarization of resin is sometimes affected on the semiconductor surface through the silicon dioxide layer.
In order to avoid the afore-mentioned drawbacks, it is considered to form a polycrystalline silicon layer doped with no impurity on the surface of the silicon semiconductor substrate. In this case, the above described drawbacks including the memory operation are improved, but another defects are caused such that resistivity is relatively low and hence leak current is increased, emitter-grounded current amplification factor h.sub.FE is low and noise is large.